1) Field of the Invention
The present invention relates to a speech coding method which executes a linear predictive coding of an input speech signal and quantizes and transmits the predictive error signal and a device employing the same, and a speech decoding method which executes an inverse quantization of a signal coded by the speech coding method and a device employing the same.
2) Description of the Related Art
In recent years, a speech coding device which compresses speech signal at high efficiency and a speech decoding device which decodes the coded signal have been demanded for intracompany communications systems, digital mobile radio systems, speech store systems, and similar systems.
Conventional speech predictive coding device multiplexes and outputs a parameter (predictive coefficient) extracted through a linear predictive coding to every frame and a parameter of an excitation signal which drives a predictive synthesis filter having the predictive coefficients as coefficients. In the decoding device, a predictive synthesis filter receives the excitation signal to reproduce a speech signal.
The all-pole type model determined by a linear predictive coding is known to be a good model of a speech producing process. For the high efficiency speech signal coding, a device which transmits linear predictive coefficients determined through a linear predictive coding analysis and parameters relating to a voice source has been widely used.
FIG. 14 shows the configuration of a conventional speech coding device. Referring to FIG. 14, the speech coding device includes a linear predictive analyzing unit 101 for performing a linear predictive coding on an input signal to calculate predictive coefficients, a predictive coefficient quantizing unit 102 for quantizing predictive coefficients calculated in the linear predictive analyzing unit 101, an inverse filter 103 for executing an inverse filtering process based on the quantizing predictive coefficient from the predictive coefficient quantizing unit 102 to determine predictive residual signals, a residual signal quantizing unit 104 for quantizing the predictive residual signal from the inverse filter 103, and a multiplexing unit (MUX) 105 for multiplexing predictive residual signal quantizing codes and predictive coefficient quantizing codes to transmit the outcome to a speech decoding device.
FIG. 15 is a diagram showing the configuration of a conventional speech decoding device. The speech decoding device, shown in FIG. 15, includes a demultiplexing unit (DEMUX) 111 for demultiplexing a signal inputted from a speech coding device through a transmission line, a predictive coefficient inverse quantizing unit 112 for executing an inverse quantizing process to predictive coefficient quantizing codes from the speech coding device, demultiplexed in the demultiplexing unit 111, a residual signal inverse quantizing unit 113 for executing an inverse quantizing process on the predictive residual signal quantizing code from the speech coding device, and a filter 114 for reproducing speech signal by executing a filtering process on the inverse-quantized predictive residual signal based on the inverse-quantized predictive coefficients.
In the speech coding device and the speech decoding device with such a conventional configuration, a signal is sampled every fixed length of span (frame) of an input signal by executing a window process, and then the sampled signal is subjected to a linear predictive coding every frame unit. Generally, the spectrum envelope every speech frame in a short time has a high correlation between adjacent frames.
In the coding, the analysis order is usually a fixed value and signals except of the analyzing span are not used. The speech decoding device receives predictive coefficient quantizing codes and predictive residual signal quantizing codes from the speech coding device to subject them to an inverse quantizing process. Filter coefficients are determined based on the resultant quantized predictive coefficients and then a speech signals are reproduced by performing a filtering process on the quantized predictive residual signals.
In order to transmit efficiently a speech signal information, there are a code excited linear predictive coding device (CELP) which vector-quantizes excitation signals and transmits its indices, and a multi-pulse excitation coding device (MPC) which models excitation signals as finite numbr of pulses and transmits optimum pulse positions and pulse amplitudes.
As described above, the predictive coefficients analyzed every frame have a high correlation between the values analyzed on adjacent frames because of a small change in the spectrum in a speech stationary period. In order to improve the quantizing efficiency of the predictive coefficients by utilizing the interframe correlation to analyzed predictive coefficients, a predictive coding and a finite state vector quantizing are known.
The conventional speech coding device and speech decoding device do not utilize any interframe correlation in the analysis stage, but utilize an interframe correlation only at the quantizing stage. Therefore, there is a disadvantage in that the interframe correlation is not used to improve prediction gain.
The change in the spectrum is large in a transient speech period, and the correlation between analyzed values in adjacent frames is low. Hence, it is required that the predictive coefficients which are subjected to a linear predictive coding are not needed to utilize the interframe correlation during the above periods